Paper is sheet material containing interconnected small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper typically is made from cellulose fibers, although occasionally synthetic fibers may be applied. Paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very low wet strength. Wet strength resin can be added to paper to produce stronger paper products. The types of wet strength resins that can be applied to paper may either be of “permanent” or “temporary” type, which are defined, in part, by how long the paper retains its wet strength after immersion in water.
Wet strength of paper is defined to be a measure of how well the fiber web holds together upon a force of rupture when in contact with water. Various techniques, such as refining of the pulp and wet pressing on the paper machine, can be used to reduce the strength loss of the paper upon wetting. The wet strength resins may improve the dry strength of the paper, as well. Wet strength improves the tensile properties of the paper both in wet and dry state by crosslinking the cellulose fibers with covalent bonds that do not break upon wetting. Wet strength is routinely expressed as the ratio of wet to dry tensile breaking force.
During the papermaking process, aldehyde functionalized polymers, such as glyoxylated polyacrylamide (GPAM), are often added to the pulp suspension before paper sheet formation to increase wet strength. Upon drying of the treated paper sheet the aldehyde functionalized polymer is believed to form covalent bonds with cellulose to increase paper dry strength and wet strength. Since the formation of covalent bond between the aldehyde functionalized polymer and cellulose is reversible in water, paper wet strength will decrease over time in water. As a result, the aldehyde functionalized polymers are also used as a temporary wet strength agent for tissue papers.
The strength performance of aldehyde functionalized polymers, such as GPAM, is known to be adversely affected by relatively high pH and high levels of alkalinity. In the absence of alkalinity, the aldehyde functionalized polymers are highly effective at acidic and neutral conditions. However, Increasing pH of the aqueous solution to a value above 7 will result in significant strength loss. With alkalinity level of 50 ppm (CaCO3) or higher, the strength performance of aldehyde functionalized polymers, such as GPAM, is impaired even at neutral pH conditions.
The negative effect of pH and alkalinity limits the application of the aldehyde functionalized polymer in many paper grades.
Papermakers often add strong acids to the pulp slurry during the papermaking process to enhance the performance of the aldehyde functionalized polymer. However, large quantity of acid is needed to lower the pH under high alkalinity conditions. Furthermore, lowering the pH of the papermaking water causes other issues, such as corrosion and compromise of process chemicals. Adding acid directly into pulp slurry results often in immediate precipitation or deposition of certain dissolved and suspended chemicals and particles. The handling of corrosive strong acids is also a safety concern for paper machine operators.
Premium bath tissue products often require relatively low dry strength and improved softness but high wet strength when in contact with water.
Tissue paper softness is a complex tactile sensation experienced by customers. This tactile sensation is a combination of several physical properties including paper surface smoothness, paper stiffness, and also paper bulk (the inverse of paper density). It has always been desired from tissue makers to continue increasing softness while achieving a particular strength target.
Chemical softeners are frequently used for improve the tactile sensation of tissue paper products. Examples of chemical softeners are waxes such as paraffin, oils such as mineral oil, fatty acids, and surfactants.
It would be highly desirable to further increase softness of a paper product while maintaining high wet strength performance when in contact with water.